The cell division cycle is a set of fundamental processes in biology, which ensure the controlled proliferation of cells. Under normal growth conditions, cell proliferation is tightly regulated in response to intra- and extracellular signals. This is achieved by a complex network of proteins that are components of signal transduction pathways. Activation of a stimulatory component or a loss of an inhibitory component can lead to the unregulated cell cycle activity, which may result in the development of cancer. Progression through mitosis is controlled by cyclin-dependent kinases, which drive cells into metaphase, and by the anaphase-promoting complex/cyclosome, a ubiquitin ligase that triggers sister chromatid separation and exit from mitosis.
To ensure proper mitosis, chromosome cohesion must be maintained until all chromosomes are attached to opposite poles of the mitotic spindle and aligned at the metaphase plate. At the onset of anaphase, the activity of separins contributes to the release of cohesins from chromosomes, allowing for the segregation of bivalents to opposite spindle poles. Separin activity is blocked by binding to a class of proteins known as securins, whose turnover at the metaphase-to-anaphase transition is triggered by the Anaphase Promoting Complex (APC). The mitotic spindle cell cycle checkpoint coordinates the timing of these events and acts as an input mechanism for DNA damage/stress pathways. Failure of this precise network leads to genomic instability and/or cell death.
Ubiquitylation enzymes provide critical signaling in a number of physiological pathways. The ubiquitin ligases provide crucial elements of specificity that direct the formation of polyubiquitin chains on protein targets, thereby marking the target for proteolytic destruction. Specificity in protein ubiquitylation derives from the substrate protein recognition by the ubiquitin ligase complex. Analysis of the SCF ubiquitin ligase has shown the utilization of substrate-specific adaptor subunits called F-box proteins to recruit various substrates to a core ubiquitylation complex.
A particularly interesting and complex ubiquitin ligase is the APC, which targets proteins containing a recognition sequence for ubiquitylation and subsequent proteolysis, and is required for mitotic progression and for exit from mitosis. This ubiquitylation reaction catalyzes the destruction of a number of mitotic substrates, including the mitotic cyclins, cyclin A and B, as well as the cell cycle regulators securin and geminin. APC activation is achieved by binding Cdc20 or Cdh1, but this and APC interactions with mitotic kinases are not sufficient to explain the timing of APC activity.
The critical role of the APC in controlling cell cycle and growth makes the further investigation of its regulation of great interest.
Relevant Literature
The Anaphase Promoting Complex and its role in cell division is reviewed by Page and Hieter (1999) Annu Rev Biochem. 68:583–609; and Zachariae and Nasmyth (1999) Genes Dev. 13(16):2039–58. A review of substrate recognition and catalysis by ubiquitin ligases may be found in Jackson et al. (2000) Trends Cell Biol. 10(10):429–39.
International patent application WO00/12679 discusses novel ubiquitin ligases.